Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Dishoeck, E.F. van

Jong, T. de

Molster, F.J.

Waters, L.B.F.M.

Leiden University Scholary Publications

Astron. Astrophys. 315, L361–L364 (1996) ASTRONOMYANDASTROPHYSICSMineralogy of oxygen-rich dust shells?L.B.F.M. Waters1;2, F.J. Molster1, T. de Jong2;1, D.A. Beintema2, C. Waelkens3, A.C.A. Boogert8, D.R. Boxhoorn2,Th. de Graauw2, S. Drapatz9, H. Feuchtgruber9, R. Genzel9, F.P. Helmich10, A.M. Heras4, R. Huygen3, H. Izumiura 7,K. Justtanont2, D.J.M. Kester2, D. Kunze9, F. Lahuis4, H.J.G.L.M. Lamers5, K.J. Leech4, C. Loup6, D. Lutz9,P.W. Morris 4, S.D. Price12, P.R. Roelfsema2, A. Salama4, S.G. Schaeidt9, A.G.G.M. Tielens11, N.R. Trams4, E.A. Valentijn 2,B. Vandenbussche3;4, M.E. van den Ancker1, E.F. van Dishoeck10, H. van Winckel3, P.R. Wesselius2, and E.T. Young131 Astronomical Institute ’Anton Pannekoek’, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands2 SRON Laboratory for Space Research Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands3 Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, B-3001 Heverlee, Belgium4 ESA Villafranca, P.O. Box 50727, E-28080 Madrid, Spain,5 SRON Laboratory for Space Research Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands6 Institut d’astrophysique de Paris, 98bis Boulevard Arago, F-75014 Paris, France7 Department of Astronomy and Earth Sciences, Tokyo Gakugei University, Konagei, Tokyo 184, Japan8 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands9 Max-Planck-Institut F̈ur Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany10 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands11 NASA Ames Research Center, Mail Stop 245-6, Moffett Field, CA 94035, USA12 Geophysics Directorate, Phillips Laboratory, 29 Randolph Rd., Hanscom AFB, MA 01731-3010, USA13 Steward Observatory, University of Arizona, Tucson, AZ 85721, USAReceived 11 July 1996 / Accepted 21 August 1996Abstract. Spectra taken with the Short Wavelength Spectrom-eter on board of the Infrared Space Observatory of dust shellsaround evolved oxygen-rich stars reveal the presence of severalemission features at wavelengths between 20 and 45m. Thesefeatures have a range of widths and strengths, but are all narrowcompared to the well-known amorphous silicate bands at 9.7and 18m. The emission peaks are tentatively identified withcrystalline forms of silicates such as pyroxenes and olivine. Theemission features tend to be more prominent for objects withcooler dust shells (T< 300 K). This may be due to an intrinsicchange in optical properties of the dust as it cools, or it may bedue to an increase in the fraction of crystalline silicates com-pared to amorphous forms as the mass loss rate increases. Theimplications for the physics of dust formation in the outflowsof cool giants are briefly discussed.Key words: infrared: stars – stars: AGB and post-AGB; massloss – planetary nebulae – dustSend offprint requests to: L.B.F.M. Waters (Amsterdam address)? Based on observations with ISO, an ESA project with instrumentsfunded by ESA Member States (especially the PI countries: FranceGermany, the Netherlands and the United Kingdom) and with the par-ticipation of ISAS and NASA1. IntroductionThe late stages of evolution of low and high mass stars arecharacterized by strong stellar winds, in which the conditionsare favourable for the formation of dust grains. These dust grainsefficiently absorb the optical and UV radiation of the central star,and re-emit this radiation at IR wavelengths, thus shifting thep ak of the energy distribution to longer wavelengths.It is believed that stellar pulsations in combination with dustformation play an important role in driving the mass loss. How-ever, current understanding of the physical processes that takeplace in the extended atmospheres of Asymptotic Giant Branch(AGB) stars and in Red Supergiant (RSG) stars is still limited. Inparticular the grain nucleation process is difficult to model fromfirst principles, and existing models lack observational data toconstrain model parameters.The Short Wavelength Spectrometer (SWS) on board of theInfrared Space Observatory (ISO) is ideally suited to study thecomposition of dusty envelopes surrounding evolved stars. Fora description of the instrument and its main features we referto de Graauw et al. (this volume) and to Kessler et al. (thisvolume). In this paper we report on first results of the SWSguaranteed time programmes on RSG, AGB stars, post-AGBstars and Planetary Nebulae (PNe). We concentrate on O-richenvelopes of representative cases, and show that the 12-45mpart of the spectrum is rich in structure. We tentatively identifythese structures with crystalline forms of silicates such as olivineL362 L.B.F.M. Waters et al.: Mineralogy of oxygen-rich dust shellsTable 1.Observing log.Name type AOT01 speed DateW Hya AGB 1 14-02-96NML Cyg RSG 4 09-01-96AFGL4106 P-AGB 2 29-02-96HD179821 P-AGB 2 09-03-96NGC6302 PN 4 19-02-96NGC6543 PN 4 12-12-95and pyroxenes. The dust features seem to occur preferentiallyin dust shells with a low colour temperature ( 300 K or less).A feature at 11.3m attributed to crystalline olivine has beenobserved in the spectra of comets (Hanner et al. 1994; Herteret al. 1987), and in several Vega stars (e.g. Fajardo-Acosta et al1993; Knacke et al. 1993).2. The observationsWe selected from the guaranteed time observing programme 6representative cases, two mass-losing objects (a RSG and anAGB star), two post-AGB stars and two PNe (Table 1). Theobservations were carried out using the SWS AOT01 full scanobserving mode, at speeds 1 to 4. Table 1 lists the observinglog. The spectra were reduced using standard procedures. Thedata were inspected for cosmic ray hits which were removed byhand. Dark subtraction was done using a linear fit to the darkmeasurements taken during the observation. The absolute fluxcalibration was obtained using the calibration files in version 4.1of the ISO-SWS off-line processing pipeline. We estimate thatthe flux calibration in the 30-45m region is accurate to 30-50percent (Schaeidt et al. 1996). In this paper we concentrate onthe 30-45m part of the scans, corresponding to band 4 of theSWS. The spectra are plotted in Figure 1, where we rebinned thedata to a resolution of 300 (/). Note that the intrinsic reso-lution of the spectrograph is significantly better than 300 (see deGraauw et al. 1996). Due to the rebinning prodecure the 30-45m spectrum of W Hya and NML Cyg do not show the emis-sion lines due rotational transitions of H2O reported by Neufeldet al. (1996) and Justannont et al. (1996a). We postpone a fulldiscussion of the spectra to a later paper. We show in Figure 1the individual data points as well as a weighted average. Thedotted line in the top left panel of Fig. 1 shows the shape of therelative spectral response curve used to divide out instrumentaleffects. Although this curve shows structure near wavelengthswhere we find some of the bumps in the programme stars, wealso find many objects that are featureless (such as W Hya) us-ing the same response function; we conclude that the bumps wereport here are real.3. DiscussionThe six programme stars discussed in thisLetterare all oxygen-rich, as evidenced from the occurence of OH maser emission, orfrom the molecular bands seen in the optical or near-IR. Also inseveral objects (W Hya, NML Cyg, AFGL 4106, HD 179821)Fig. 1.SWS 30-45m spectra of the programme stars (Table 1). Fluxis in Jythe 9.7m silicate band (not shown here) is seen either in emis-sion or absorption, pointing to an O-rich chemistry in the shell.The programmestars show a remarkably rich spectrum between30 and 45m, with several bumps with a variety in strength andshape. Also, in several objects we find a plateau between 31 and36-37m. W Hya however shows an almost featureless spec-trum (apart from the water lines). Table 2 gives a summary ofthe bumps with their wavelength and line over continuum ratio.In Fig. 1 we have indicated the position of the strongest featureswith tick marks in the upper panels.The bumps are strongest in the post-AGB stars and PNe,while they are weak in NML Cyg and virtually absent in W Hya.This suggests that the appearance of the bumps is somehow re-lated to the colour temperature of the dust shell, the bumps beingabsent in ’warm’ dust shells, while they are prominent in dustshells with colour temperature less than about 300 K. Inspectionof the spectra of other stars observed with SWS shows a similartrend. In mass-losing RSG or AGB stars the colour temperatureof the dust is a measure of the mass loss rate of the star, whilein post-AGB stars and PNe it is a measure of the time whichhas passed since the star left the AGB. This suggests a relationbetween the occurence of the bumps either with grain formationconditions (high mass loss versus low mass loss) or with ageand temperature (post-AGB and PNe; see also below).L.B.F.M. Waters et al.: Mineralogy of oxygen-rich dust shells L363Table 2.wavelength and possible identification of featuresName  FWHM Ip/Ic flux W/m2 identm m 10 14W Hya 31.7 0.1 0.9 1.06 9.0 uNML Cyg 30.5 0.1 0.6 1.02 20 u32.95 0.01 0.83 1.06 71 f?33.75 0.01 0.59 1.05 35 o40.40 0.03 1.2 1.05 37 cp43.19 0.04 0.61 1.026 7 iAFGL4106 30.52 0.03 0.8 1.04 18 u32.83 0.01 0.4 1.06 23 f?33.50 0.03 1.0 1.06 16 o36.51 0.02 0.4 1.04 6 u40.1 0.1 1.6 1.07 27 cp43.0 0.2 1.4 1.04 13 iHD179821 30.59 0.01 0.28 1.028 2 u32.82 0.01 0.4 1:082 5 f?33.49 0.01 1.0 1:082 12 o31.7-37.1 0.3 23 plateau140.39 0.01 0.8 1.06 7 cp43.1 0.1 1.24 1.08 15 iNGC6302 30.52 0.04 0.35 1.06 8 u32.80 0.02 0.5 1.152 60 f?33.70 0.01 0.95 1.212 19 o31.7-37.2 0.2 72 plateau140.46 0.05 1.10 1.15 31 cp41.54 0.02 0.34 1.045 3.5 u43.1 0.1 1.51 1.10 30 iNGC6543 31.4-37.5 0.3 22 plateau133.38 0.03 1.6 1.122 11 o43.34 0.04 1.22 1.08 3.0 inotes to table 2: o = crystalline olivine; cp = clino-pyroxene; f =forsterite; i = crystalline ice; u = unidentified.1 plateau isunidentified;2 Ip/(Ic + plateau)The width and shape of the bumps, which are all resolvedat a resolution of 300, suggest that they are due to vibrationalor bending modes in solid state material (grains). We have triedto identify the carriers of the bumps by comparing publishedlaboratory spectra of O-rich grain material (silicates, oxides) tothe observed spectra. It is clear from these (preliminary) com-parisons that we are dealing with amixtureof materials, i.e.no single component that we are aware of can fit the observedspectra. This may not be surprising since the relative strengthof some bumps seems to correlate while others do not, alsopointing to a mixture of carriers.The most promising candidates for the carriers of the bumpsare crystalline silicates, such as Olivine and Pyroxenes, withvarious mixtures of Fe and Mg (e.g. Jäger et al. 1994; Koike etal. 1993). We have indicated possible identifications in Table 2.Amorphous materials do not show prominent structure beyond20m (e.g. J̈ager et al. 1994). We point out however that the 10m spectrum of NML Cyg has a shape which fits amorphoussilicates, which suggests that at least in that object there is a sig-Fig. 2. Laboratory spectra of crystalline silicates, taken from Koikeet al. (1993). Solid line: forsterite; Dotted line: olivine; Dashed line:clino-pyroxene.nificant amount of amorphous material present. This may alsobe the case for the other objects (see e.g. Guertler et al. 1996).We conclude that the dust shells of our programme stars consistof a mixture of amorphous and crystalline silicates. Crystallineolivine shows peaks at 11.3 and 23.8-25.9m in addition to apeak at 33.8m (Koike et al 1993). A 23.5m peak is seen inAFGL 4106, HD 179821, NGC6543, NGC6302 and possiblyin NML Cyg, but not in W Hya, which is close to that expectedfor olivine. The spectra of several objects show structure around11.3m, i.e. where emission from crystalline silicates is ex-pected. However this may be instrumental and not intrinsic. InFigure 2, we show the extinction curves for several materialsthat were measured in the lab by Koike et al. (1993).We also considered the possibility that the bumps are dueto metal oxides, such as Al2O3, and Fe-Mg oxides. Evidencefor the occurence of oxides in oxygen-rich dust envelopes isgrowing. The LRS spectra of some Mira variables with a broadpeak between 12 and 13m were interpreted by Vardya et al.(1986) as due to grains containing aluminum oxide. Interest-ingly, the occurence of the 12-13m peaks seems to be relatedto the shape of the optical light curve, suggesting a relation be-tween the physical conditions in the dust-forming region andthe kind of dust that condensates (Onaka et al . 1989) ¿From are-classification of IRAS LRS spectra Goebel et al. (1989) finda class of objects with bumps at 11+, 13.1 and 19m, whichthey interpret as due to metal oxides. Henning et al. (1995) pub-lish new optical constants for Mg-Fe oxides and find that thesematerials show structure in the 15-24m wavelength region. Its possible that metal oxides contribute to the bump seen at 23.5m. However, we are not aware of laboratory measurements ofoxides that show structure between 30 and 45m. We point outthat the list of spectral features presented in thisLetter is pre-liminary and certainly not complete for wavelengths shortwardof 30m. For example, NML Cyg shows a bump near 20mwhich could be due to oxides.L364 L.B.F.M. Waters et al.: Mineralogy of oxygen-rich dust shellsA third possibility for the origin of the bumps are (crys-talline) ices. Emission from ice in the 40-70m spectrum ofthe ’Frosty Leo’ and other cool oxygen-rich envelopes was re-ported by Omont et al. (1990). Laboratory spectra of crystallineH2O ices show a narrow emission at 44m (Bertie et al. 1969)as well as a broader feature at 62m. Indeed one of the featureswe find peaks near 43m (Table 2). We have tentatively iden-tified the 43m bump with crystalline ice. Confirmation of thisidentification may come from LWS spectra which then shouldshow the 62m feature.If the identification of the bumps with crystalline material iscorrect, the correlation with dust temperature seems puzzling.It is believed that silicates condense in dusty outflows withamorphous structure, as evidenced by the ubiquitous broad andsmooth appearance of the 9.7 and 18m silicate bumps (e.g.Bedijn 1987). Conversion of this material to crystalline formsrequires restructuring of the lattice which can be done by heat-ing the grains. However, heating of dust grains is not expectedto occur in the post-AGB phase and indeed is not observed.Rather, cooling of the dust shell as it expands is observed. It isimportant to realise however that dust in post-AGB stars repre-sents a final burst of very high mass loss (10 5 to 10 4 M/yr)and so the conditions in the dust forming region during this veryhigh mass loss phase may have been similar to that seen in thereddest AGB stars or RSG we observe now. Indeed, NML Cyg,the reddest mass-losing object in our sample, shows the bumpsin emission. Therefore it is possible that the conditions in thedust forming regions of AGB stars and RSGs with extreme massloss rates and high wind densities allow the formation of crys-talline silicates. This could occur if the grains stay warm in thishigh-density region for a relatively long period of time, i.e. donot cool rapidly after formation.It is also possible that the intrinsic properties of the dustgrains change with temperature. There is some evidence frommodel fitting of cool oxygen-rich dust shells that the strengthof the 18m bending mode of SiO increases with decreasingtemperature (Bedijn 1987; Justtanont et al. 1996b). Note thatthe observed ratio of the 18 to 9.7m silicate features dependson the temperature of the dust shell as well as on the opticalconstants (see e.g. Ossenkopf et al. 1992). It is not clear howeverhow important this effect is for the 30-45m features.It is interesting to point out that bumps similar to the onesreported here have also been found in cool dust shells surround-ing LBV’s (Lamers et al., this volume) and in young HerbigAe/Be stars (Waelkens et al., this volume). All these objectsare believed to be oxygen-rich. Prior to ISO, structure in the 30m spectra of O-rich stars was, to our knowlegde, not reported.However the 23.5m feature was previously found in spectraof comet Halley (Herter et al. 1987) and was identified withcrystalline olivine.The dust features reported in this paper can be added to thegrowing list of structures found in spectra of oxygen-rich stars.Goebel et al. (1994) report on the discovery of 7.15m emissionin O-rich shells, and they also list features at 10, 13.1, 18 and19.7m. These features are attributed to metal oxides. It is clearthat these new observations force us to revise the picture of astandard mix of silicates (’astronomical silicates’, e.g. Draine &Lee 1984). The full wavelength coverage of the SWS and LWSspectrographs allow us to make a much more detailed inventoryof the composition of dust shells in evolved stars, i.e. to performmineralogy of these objects.Acknowledgements.It is a pleasure to thank Drs. Koike, Dorschner,Henning and Lynch for sending us information on laboratory spec-tra of silicates and oxides, and Dr. Henning for critically reading themanuscript. LBFMW acknowledges financial support from the RoyalNetherlands Academy of Arts and Sciences, and from an NWO ’Pio-nier’ grant. FJM acknowledges support from NWO grant 781-71-052.ReferencesBedijn, P.J.: 1987, A&A 186, 136Bertie, J.E., Labbe, H.J., Whalley, E.: 1969, J. Phys. 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Mineralogy of oxygen-rich dust shells

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Mineralogy of oxygen-rich dust shells

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